A primary factor affecting the distance from a central office that digital subscriber line (DSL) service can be provided is the noise environment experienced by the line over which the service is offered. Therefore, it is important in deploying DSL service to have an accurate estimate of the noise value. The greater the noise value, the less distance from the central office that DSL service maybe provided. If the estimates of the noise values are too high, a high failure rate will occur as service is installed beyond what is a reliable distance. estimates of the noise are too high, potential subscribers who could have service reliably provided would be denied.
Typically there are several sources of noise “near end crosstalk (NEXT), far end crosstalk (FEXT), and ambient noise (AM Radio, etc.).” Crosstalk (both near and far) is noise generated when a signal on one twisted pair electromagnetically couples to another. The strength of this coupling is significant only when the two twisted pairs are within the same binder group. As the name indicates, NEXT is that noise which couples from a transceiver (a DSL modem or any other signal generator) at the near end of the cable and FEXT is that noise which couples from a transceiver at the far end of the cable.
Typically, the magnitude of any NEXT is much greater than FEXT since the source of disturbance is closer to the affected transceiver. A variety of factors may affect the level of any NEXT, including the frequency of the signal, the number of pairs which have disturbing signals on them, the type of cable in which the signal is present, and the specific pairs within the cable.
A current model which exists and has been adopted by the ANSI T1E1 standards body, estimates the transfer functions for the noise at the 1% worst case level, meaning that in only 1% of the cases would the noise be worse than this level, in 99% of the cases the noise will not be as bad. The transfer function is simply the factor by which the strength of the original signal is multiplied to obtain the strength of the noise. The ANSI model depends on the frequency of the signal and the number of disturbing pairs. The signal strength is assumed identical in each of the disturbing pairs. An opinion held by some is that the current model is conservative and over estimates the noise.